1. Field of the Invention
The present invention relates to inductive displacement sensors. Such devices may be used for non-contact sensing of either angular or linear displacement.
2. Summary of the Prior Art
The best established non-contact inductive angle sensor for continuous 360.degree. rotation applications is the brushless resolver. It is an inherently complex electrical machine, comprising a multipole wound rotor and stator, with a rotary transformer to couple the excitation a.c. supply to the rotor. There are two stator windings which have voltages induced in them which vary sinusoidally with angle. Being relatively displaced by 90.degree., they correspond to sine and cosine, from which the shaft angle can be calculated ratiometrically. As this technology originated about the time of World War II for military and naval fire control systems, the sensors and electronic signal processing hardware are well developed. Recent advances have now brought the cost of the electronics much lower, single-chip Resolver-to-Digital converters becoming commonplace. But the resolver itself remains complex and expensive.
We have previously developed alternative forms of sensor, e.g. as disclosed in W088/06716 and W090/04153. The former discloses devices employing two members which are relatively displaceable along a linear or circular path. One member provides a plurality of coil portions along the path. Their individual inductances are dependent on the configuration of the second member. For example, the first member may be an annular stator with inward radial projections on which the coil portions are wound, alternatively in different senses so that the flux path tends to loop in and out. The second member may be an annular rotor, one section being ferromagnetic and the other section being non-ferromagnetic and/or having a conductive screening layer. Alternatively the second member may be mercury half-filling an annular conduit which surrounds a static ferromagnetic core. A particular characteristic is that for any relative position of the members, half the coil portions have a different inductance from the other half. Generally the polarity of the coil portions alternate: that is, the array is heteropolar. In contrast WO90/04152 discloses essentially homopolar devices. The devices can be quite similar to the heteropolar analogues. Thus displacement along a linear or rotary path causes relative displacement of a pair of elements that confront each other across the path. One element provides a series of coil portions while the other has a portion which increases the inductance of the fraction of the coil portions that it lies adjacent at any instant. The coil portions are homopolar. Typically each coil portion has an axis which intersects the path, and all are wound in the same sense about their axes. They are generally connected in series as a single winding on a core. The core has unwound portions for providing a flux return path.
Both the hetero- and homopolar devices are designed to give substantially linear outputs over useful working ranges. Clearly, linear outputs are easy to process. In a homopolar angle sensor as disclosed in WO90/04152, the basic output is derived from the centre tap of a single auto transformer winding. The span of the screen or equivalent ferromagnetic rotating element and also the half-winding is 180.degree.. There is inherent rounding of the characteristic at the transition points, but the output is largely linear over 180.degree. rotation. The same is true of devices for reduced angle sensing in which the number of sets of windings and rotor screens is multiplied, e.g. two diametrically opposite 90.degree. screens affecting two pairs of 90.degree. span half-windings give a linear characteristic over 90.degree. rotation.